1. Field of the Invention
The present invention relates to a substrate processing method and a substrate processing apparatus applied for removing, from the surface of a substrate, resist no longer required. Examples of the substrate to be processed include a semiconductor wafer, a liquid-crystal-display-device substrate, a plasma-display substrate, an FED (Field Emission Display) substrate, an optical-disk substrate, a magnetic-disk substrate, a photomagnetic-disk substrate, a photomask substrate and the like.
2. Description of Related Art
The processing steps of a semiconductor device include, for example, a step of locally implanting ions, such as phosphorus, arsenic, boron or the like, onto the surface of a semiconductor wafer (hereinafter simply referred to as a wafer).
Prior to the ion implantation step, there is formed, on the surface of a wafer, a resist pattern for masking portions thereof to which ions should not be implanted. The resist pattern is formed by applying a resist on the entire wafer surface to form a resist film thereon and then by selectively removing (through light-exposure and development) the resist film. After the ion implantation step, the resist on the wafer surface becomes unneeded. Therefore, there is executed a processing for removing, from the wafer surface, the resist thus no longer required.
In a typical processing for resist removal, oxygen plasma is irradiated onto the wafer surface to ash the resist thereon. Then, a chemical liquid such as an APM (ammonia-hydrogen peroxide mixture) or the like is supplied onto the wafer surface, thereby to remove the ashed resist, thus achieving the removal of the resist from the wafer surface.
However, the irradiation of oxygen plasma for ashing the resist, results in damage to those portions of the wafer surface not covered with the resist (e.g., oxide film portions exposed from the resist).
Therefore, another method recently becomes the object of public attention. According to this method, the resist ashing is not executed, but an SPM (sulfuric acid/hydrogen peroxide mixture) which is a mixture of a sulfuric acid with a hydrogen peroxide solution is supplied onto the wafer surface to strip and remove the resist by the strong oxidizing force of the peroxomonosulfuric acid (H2SO5) contained in this SPM.
When the resist is present in the vicinity of the peripheral end face of the wafer, there is a likelihood that the resist in the vicinity of the wafer peripheral end face comes in contact with a hand for wafer delivery to cause the resist to stick to the hand, when the wafer is delivered, thereby to cause a pollution of the wafer. Accordingly, after the resist film has been formed on the wafer surface, a rinse liquid is supplied to the vicinity of the peripheral end face of the resist film to flush away the resist film in the vicinity of the wafer peripheral end face. Therefore, as shown in FIG. 3, the resist on the wafer peripheral edge portion is so inclined as to be reduced in thickness in the direction nearer to the wafer peripheral edge.
Accordingly, when a high-dose ion implantation is conducted, the resist at the center portion of the wafer is hardened only at the surface thereof, while the resist at the peripheral edge portion of the wafer is hardened in its entire thickness at its portion having a reduced thickness, as shown by the broken line in FIG. 3. Moreover, in the positive-type resist, for simplification of the production process, the wafer peripheral edge portion (non-device forming region) is not exposed to light and the resist on the wafer peripheral edge portion is therefore not patterned. As a result, the SPM enters the gaps in the resist pattern at the wafer center portion, so that the hardened layer formed on the surface can be stripped together with the resist which is below the hardened layer and not hardened. At the wafer peripheral edge portion, however, the resist has no gaps into which the SPM enters, and the SPM therefore does not penetrate the resist. This disadvantageously prevents the hardened resist from being removed. In particular, when the ions to be implanted into the wafer are arsenic ions, the resist is hardened considerably, thereby further making it difficult to remove the resist.